Of all the creatures in Pandaria, none have been quite so mysterious as the mogu. From day one they were presented as one of the villains in the saga of Pandarian history -- and although the days of the mogu empires were long over, their legacy lived on. Mogu architecture, mogu statues, mogu ruins, they all littered the landscapes of where we leveled. To the pandaren, the mogu were a threat, but one that had long since died out, leaving the race as little more than scary tales to tell the children at night.
Until Mists of Pandaria, and the arrival of the Alliance and Horde. With the sudden uprising of the mantid, the release of the sha, and the frightened movement of the yaungol, the pandaren had more than enough to contend with. The sudden explosion of mogu activity was just another addition to the pile -- and the appearance of the Zandalari as allies made the reappearance of this ancient threat even more dire.
But who are the mogu? Until patch 5.2, that mystery hadn't been fully defined. And it still may not be fully defined, but at least we have a slightly clearer picture.
Please note that today's Know Your Lore contains some spoilers for patch 5.2 Lorewalkers content.